Magnesia partially stabilized zirconia wire drawing die assembly

ABSTRACT

The present system provides a wire drawing die assembly having a magnesia oxide partially stabilized zirconia die or nib for contacting or reducing the diameter of a wire during a draw. The magnesia partially stabilized zirconia has a density of approximately 5.75-5.90 g/cc; a fracture toughness of approximately 10-12; and a flexural strength of approximately 575-700 MPa.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wire drawing assemblies, and particularly, to a wire drawing die assembly having a partially stabilized zirconia die and, more particularly, to wire die assembly having a magnesia partially stabilized zirconia nib, which can be interchangeably and operably located within a casing.

2. Description of Related Art

Wire is often formed by drawing a relatively large diameter length of metal through a die, wherein the die is sized to reduce the diameter of the wire. By passing the wire through successively smaller dies, the wire can be shaped to any of a variety of relative small diameters.

Usually the drawing process can employ as many as one to fifteen die assemblies, wherein each die assembly is used to continually reduce the cross-sectional diameter of the wire through the drawing operation. However, during operation, if one of the dies overheats, chips, develops any flaw or defect or just wears so as to fall outside of tolerance, the die will impart a defect in the wire being drawn, thereby rendering the drawn wire worthless and unusable.

Often, a lubricant is used to assist the wire to be forced or pulled through the die opening and to provide for longer die life or to attempt to cool the die continuously as the wire is pulled or forced through.

The dies are often formed of carbides, such as tungsten carbide, tantalum carbide or titanium carbide set forth in U.S. Pat. Nos. 4,161,415 and 3,469,436.

U.S. Pat. No. 4,038,858 discloses an aluminum oxide ceramic die for drawing wire. However, use of this type of ceramic dies has been limited because of the brittle nature of the materials used and has often resulted in very short surface life for such dies which this causes increased costs throughout the industry.

Therefore, the need remains for a wire drawing die assembly that can increase system run time, while reducing imparted flaws or defects in the drawn wire.

BRIEF SUMMARY OF THE INVENTION

The present system provides an apparatus including a wire drawing die having a forming passage, the wire drawing die being formed of partially stabilized zirconia. In one configuration of the system, the partially stabilized zirconia is a monolithic partially stabilized zirconia. In a further configuration of the system, the partially stabilized zirconia is a magnesia partially stabilized zirconia.

It is contemplated the wire drawing die can be configured as a nib. Further, the wire drawing die can be employed with a casing having a through aperture, the through aperture sized to receive the wire drawing die. The casing can include a shoulder projecting inwardly into the through aperture, wherein the shoulder is sized to contact the wire drawing die.

The present disclosure also provides for a method including the steps of reducing a diameter of a ferrous wire by passing the wire through a wire drawing die formed of a monolithic partially stabilized zirconia; and collecting a reduced diameter wire after passing through the wire drawing die. It is contemplated the wire drawing die can be formed of magnesia partially stabilized zirconia.

A further method is provided as an improved method of drawing a surface treated wire to the reduced diameter, by passing a length of the surface-treated wire through a die formed of a monolithic magnesium oxide partially stabilized zirconia. A forming surface of the monolithic magnesia partially stabilized zirconia wire drawing die can be formed of can be selected to pass the wire substantially free of imparted striations. It is contemplated the monolithic magnesium oxide partially stabilized zirconia can be monolithic.

Further, the partially stabilized zirconia wire drawing die can be formed as a nib sized to be cooperatively received within a standard metal casing, thereby providing compatibility with installed processing lines and facilitating heat dissipation from the ceramic nib to the casing.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a top plan view of a wire drawing die assembly.

FIG. 2 is a cross sectional view of the wire drawing die assembly of FIG. 1 taken along lines 2-2.

FIG. 3 is a perspective view of the wire drawing die assembly of FIG. 1.

FIG. 4 is a top plan view of a casing of the wire drawing die assembly of FIG. 1.

FIG. 5 is a cross sectional view of the casing of FIG. 4 taken along lines 5-5.

FIG. 6 is a perspective view of the casing of FIG. 4.

FIG. 7 is a top plan view of the nib of the wire drawing die assembly of FIG. 1.

FIG. 8 is a cross sectional view of the nib of FIG. 7 taken along lines 8-8.

FIG. 9 is a perspective view of the casing of FIG. 7.

FIG. 10 is a top plan view of an alternative configuration of a wire drawing die assembly.

FIG. 11 is a cross sectional view of the wire drawing die assembly of FIG. 10 taken along lines 11-11.

FIG. 12 is a perspective view of the wire drawing die assembly of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

The present system encompasses wire drawing die assemblies and referring to FIGS. 1 and 10-12, a partially stabilized zirconia wire drawing die assembly 10.

In one configuration, shown in FIGS. 1-9, the wire drawing die assembly 10 includes a wire drawing die 20 and a casing 40, wherein the wire drawing die is configured as a nib 30 retained by the casing, as seen in FIGS. 1-3.

Generally, the wire drawing die 20 has a generally cylindrical body 22 having a central forming passage 23, typically having a generally circular cross-section. The forming passage 23 includes an approach (or entrance) zone 24, a reduction zone 26, a bearing zone 28, and optionally a back relief zone 29.

Although the external shape of the wire drawing die 20 can be any of a variety of configurations, in order to provide interchangeability with existing casings, the nib configuration of the wire drawing die 20 has a circular cross-section thus providing a generally cylindrical outer surface.

The approach zone 24 has a generally tapered side walls opening to an entrance face or port 25 on an entrance (upstream) side of the wire drawing die 20.

The reduction zone 26 is located immediately down stream of the approach zone 24, and has tapered side walls sized to impart a reduction in the cross section of the passing wire.

The bearing zone 28 is located immediately down stream of the reduction zone 26, and has a cross sectional area substantially equal to the minimal cross sectional area of the reduction zone 26. The cross sectional area of the bearing zone 28 may be substantially equal and constant throughout the length with the bearing zone being generally cylindrical. The bearing zone 28 is typically straight sided and is used to set the cross section of the passing wire after reduction in the reduction zone 26. Alternatively, the bearing zone 28 can have a varied diameter along the length.

Optionally, the back relief zone 29 may be defined by an increasing cross sectional area from the bearing zone 28.

In operation, deformation of the wire is typically initiated in the approach zone 24 at the point where a diameter of the approach zone meets the reducing diameter of the reduction zone 26 with final shaping and elongation of the wire typically occurring in the reduction zone. The downstream bearing zone 28 sets the cross section formed by the reduction zone 26.

The angles, lengths, ratios of the zones of the wire drawing die 20 are not critical to the present preferred embodiments and are determined by numerous factors including, but not limited to the type of wire being drawn, the speeds to be run and necessary change in cross sectional area.

In the configuration of the wire drawing die 20 as the nib 30, the nib is slidingly received within the casing 40. By forming the wire drawing die 20 as the nib 30, a reduced volume of material is required to define the contacting (forming) surfaces with the metal to be shaped and the casing 40 can be formed from more economic materials.

Typically, the casing 40 includes a through aperture 41 and an inwardly projecting shoulder 42 such that the nib 30 can be operably disposed within the through aperture and contact the shoulder to operably locate the nib relative to the casing. Alternatively, the through aperture 41 of the casing 40 can include a slight taper to engage the outer surface of the nib 30 and thereby operably retain the nib.

The casing 40 can be formed of from carbon steel, to reduce cost and provide for heat dissipation. In addition, a shrink fit can be formed between the casing 40 and the nib 30. For example, the casing 40 is heated and the nib set into the aperture 41 to contact the shoulder 42. Upon cooling of the casing 40, the contraction of the steel casing clamps the nib 30 for operation.

In a further embodiment, it is contemplated a bushing can be sized to be operatively received within the through aperture 41 of the casing 40, wherein the bushing then receives the outer dimension of the nib 30.

In the nib configuration of the wire drawing die 20, the thermal characteristics of the partially stabilized zirconia wire drawing die relative to the casing can be matched. That is, the thermal expansion of MgPSZ is very similar to carbon steel and therefore there is a reduction in cumulative thermal differential when compared with other options. For example, the thermal expansion of MgPSZ (20° C.) is 10*10⁻⁶/° C. (m/m° C.), where the thermal expansion of carbon steel (20° C.) is 9.9*10⁻⁶/° C. Therefore, the partially stabilized zirconia wire drawing die 20 can be operably retained within a carbon steel casing 40, such that the stresses imparted by the thermal conditions during operation do not negative impact the functioning of the partially stabilized zirconia wire drawing die or the casing.

Referring to FIGS. 10-12, the wire drawing die 20 can be a one piece or monolithic construction of the MgPSZ, or equivalent material, wherein the die provides the central forming passage 23 including the approach (or entrance) zone 24, the reduction zone 26, the bearing zone 28, and optionally the back relief zone 29. In addition, the wire drawing die 20 can be sized for functionally replace the casing 40.

The wire drawing die 20 is formed of partially stabilized zirconia. Partially stabilized zirconia (PSZ) is a mixture of several polymorphs of zirconia that develop as a consequence of starting MgO/ZrO₂ chemistry (typically in the range of 3.2-3.5 mol % MgO) and the firing cycle (peak and soak temperature with subsequent cooling) used to densify the ceramic. The microstructure that develops consists of domains (20-80 microns) that are intergrowths of cubic, tetragonal, and monoclinic crystalline phases.

PSZ is a transformation-toughened material. Microcrack and induced stress may be two explanations for the toughening in partially stabilized zirconia. The microcrack explanation depends upon difference in the thermal expansion between the cubic phase and monoclinic (or tetragonal)-phase particles in the PSZ. This difference creates microcracks that dissipate the energy of propagating cracks. The induced stress explanation depends upon the tetragonal-to-monoclinic transformation. Energy associated with an initiated and traveling crack is “absorbed” as it encounters the tetragonal phase causing it to transform to a larger volume monoclinic phase and thus be blunted.

PSZ can be generally cream colored with approximately 10% MgO.

While it is contemplated the PSZ material can be reformed or stressed before or after it is formed into the die configuration, such reformation or stressing is an optional processing step.

Representative property values for PSZ include:

Property* Value General Chemical Formula ZrO₂—MgO Mechanical Density 5.75-5.9 gm/cc Hardness 1120-1300 Knoop- Tensile Strength 65 kpsi (448 MPa) Modulus of Elasticity 29-30 × 10⁶ psi Flexural Strength 100 kpsi (689 MPa) Compressive Strength 268 kpsi (1847) Poisson's Ratio 0.23-0.31 Fracture Toughness 10-12 MPa m^(1/2) Electrical Dielectric Strength 250 ac V/mil Volume Resistivity >10¹⁴ ohm-cm Thermal Coefficient of Thermal Expansion 10.1 × 10⁻⁶/° C. Thermal Conductivity 1.8-3 W/mK Specific Heat 0.10 cal/g ° C. Maximum Working Temperature 1100° C. Shock Resistance 400° C. Diff. All properties are at room temperature unless otherwise noted.

As to other materials, cubic zirconia in Table above (the 10 mol % Y₂O₃) is not a structural ceramic—it is an electronic ceramic and YTZP (typically at 3 mol % Y₂O₃) typically has a fracture toughness in the range 5-8.

PSZ Properties

Melting Temperature, ° C. ~2800 Range of working temperatures, ° C. −140-+2400 Thermal extension index, grad-1 10-11 × 10⁻⁶ Thermal conductivity, W/m * K 7.8 Tensile strength in bending, MPa 800-1200 Tensile strength in compression, MPa 2300-3700 Micro-hardness, GPa 11.8-15.08 Elastic modulus, GPa 180-372 Poisson coefficient 0.26-0.36 Tensile relative deformation in bending 0.07-0.45 * 10−2 Crack resistance, MPa * m0.5 0.56-15 Wear-out rate 2.5-3 × 10⁻⁹ Friction index 0.27-0.34

In one configuration, the wire drawing die 20 is formed of PSZ, having a density of approximately 5.75-5.90 g/cc; a fracture toughness of approximately 10-12; and a flexural strength of approximately 575-700 MPa.

In selected configurations, the Mg PSZ is formed by the method or steps of conventional ceramic processing methods/equipment combined with a firing cycle after pressing the powder into a formed shape like a cylinder with a given ID. The sequence is: react starting raw materials (MgO-based and ZrO₂-based; i.e. precursors can be used) at elevated temperature; mill the reacted powder to submicron particle size; form a slurry with deionized water, binder, and powder; spray dry the slurry to obtain a flowable, bindered powder; press powder to desired shape—e.g. with any forming equipment such as isostatic press, uniaxial press; fire the shaped powder at elevated temperature to densify the material; machine as needed/required.

It is contemplated that the wire drawing die 20 can be configured to replace the entire casing 40. As the PSZ material of such wire drawing die 20 has the above recited thermal conductivity, such wire drawing dies can be incorporated into the processing line without requiring traditional cooling systems or cooling jackets.

Further, the present PSZ wire drawing die 20 can maintain surface properties of the passing wire. For example, the Mg PSZ wire drawing die 20 does not impart striations to the passing wire as compared to a comparably sized and located tungsten carbide or aluminum oxide die.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made to it without departing from the spirit and scope of the invention. 

1. An apparatus comprising: (a) a wire drawing die having a forming passage, the wire drawing die being partially stabilized zirconia.
 2. The apparatus of claim 1, wherein the partially stabilized zirconia is a magnesia partially stabilized zirconia.
 3. The apparatus of claim 1, wherein the partially stabilized zirconia is a monolithic magnesia partially stabilized zirconia.
 4. The apparatus of claim 1, wherein the wire drawing die is configured as a nib.
 5. The apparatus of claim 1, further comprising a casing having a through aperture, the through aperture sized to receive the wire drawing die.
 6. The apparatus of claim 1, further comprising a casing having a through aperture, the through aperture sized to receive the wire drawing die, the casing including one of (i) a shoulder projecting inwardly into the through aperture, the shoulder sized to contact the wire drawing die and (ii) a taper sized to contact the wire drawing die.
 7. The apparatus of claim 1, further comprising a casing having a through aperture and a bushing sized to be received within the through aperture, the bushing sized to receive the wire drawing die.
 8. The apparatus of claim 1, wherein the partially stabilized zirconia is monolithic.
 9. A method comprising: (a) reducing a diameter of a metal wire by passing the wire through a wire drawing die formed of a partially stabilized zirconia; and (b) collecting a reduced diameter wire after passing through the wire drawing die.
 10. The method of claim 9, wherein the wire drawing die is magnesia partially stabilized zirconia.
 11. The method of claim 9, wherein the metal wire is ferrous.
 12. The method of claim 9, wherein the wire drawing die is monolithic.
 13. An improved method of drawing a surface treated wire to the reduced diameter, the improvement comprising: (a) passing a length of the surface-treated wire through a die formed of a magnesium oxide partially stabilized zirconia.
 14. The improved method of claim 13, wherein the magnesium oxide partially stabilized zirconia is monolithic.
 15. The improved method of claim 13, further comprising providing a forming surface of the magnesium oxide partially stabilized zirconia to pass the wire substantially free of striations. 